Shell and tube heat exchanger with improved anti-fouling properties

ABSTRACT

A shell and tube heat exchanger comprising: a shell having an inlet end-cap attached to a first end of the shell, wherein an outlet end-cap is attached to a second end of the shell and a tube bundle being housed within the shell, said tube bundle including a plurality of parallel-spaced tubes that traverse the interior of shell from a first end to a second end of the tube bundle, and wherein a plurality of baffles are arranged within the shell supporting the parallel-spaced tubes of the tube bundle. At least a part of the shell and tube heat exchanger is provided with a coating comprising silicon oxide, SiO x .

TECHNICAL FIELD

The present invention refers generally to shell and tube heat exchangersallowing a heat transfer between two fluids at different temperature forvarious purposes. Specifically, the invention relates to a shell andtube heat exchanger which has been coated for improving anti-foulingproperties and has in some embodiments been given predetermined,structural properties for ensuring that the coating remains on the shelland tube heat exchanger when it is used.

BACKGROUND ART

In many industrial processes fouling of heat transfer equipment is ofconcern. In order to keep a satisfying performance of the equipmentregular service and cleaning it is necessary to remove build up ofdeposits on the heat transfer surfaces. The deposits arise e.g. from thefluids in the equipment, microbial growth and/or dirt.

Shell and tube heat exchangers may over time get fouled which leads to adecreased heat transfer and increased pressure drop, and thus leads toan overall reduced performance of the heat exchanger. Depending e.g. onthe fluids used the heat exchanger may be seriously fouled and difficultto clean, thus requiring strong detergents and/or powerful mechanicalcleaning over a substantial time period in order to restore theperformance of the heat exchanger. The cleaning may both be timeconsuming and costly. Also, the process to which the shell and tube heatexchanger is connected to may have to be shut down during said cleaning.

The shell and tube heat exchangers are made of metals which have a highsurface free energy that results in most liquids easily wetting thesurfaces.

Also, when heat exchanger surfaces are produced the forming operation ofthe metal increases the surface roughness which often is associated withfaster build up of fouling deposits.

GB2428604 discloses provision of a coating on shell and tube heatexchangers to reduce fouling.

US20080073063 discloses a shell and tube heat exchanger coated with alow surface energy material to reduce fouling.

It would be desirable to find new ways to ensure less fouling of heatexchangers and their surfaces in order to keep the heat exchangersrunning for longer time periods. Also, a reduced shut down time forprocesses where shell and tube heat exchanger are involved would bedesirable.

A problem encountered with presently known antifouling coatings is thepoor wear resistance of the coatings in applications with abrasive heatexchanging media, e g sand or other particulate material which entersthe shell and tube heat exchanger with the heat exchanging fluids.Furthermore, cracks in the coating may occur due to torque and tensionforces acting in the shell and tube heat exchanger in applications underhigh pressures.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved surfacesfor a shell and tube heat exchanger, which show a reduced fouling of thesurfaces when in use in a shell and tube heat exchanger. Another objectis to achieve surfaces for a shell and tube heat exchanger havingantifouling properties which are wear resistant in abrasive environmentsand have high resistance against formation of cracks.

This object is achieved by a shell and tube heat exchanger comprising ashell having an inlet end-cap attached to a first end of the shell,wherein an outlet end-cap is attached to a second end of the shell and atube bundle being housed within the shell, said tube bundle including aplurality of parallel-spaced tubes that traverse the interior of shellfrom a first end to a second end of the tube bundle, and wherein aplurality of baffles are arranged within the shell supporting theparallel-spaced tubes of the tube bundle. The shell and tube heatexchanger is provided with a coating comprising silicon oxide, SiO_(x),having an atomic ratio of O/Si >1, a content of carbon ≧10 atomic % anda coating layer thickness of about 1-30 μm, which coating was preparedby sol-gel processing and applied to at least a part of the shell andtube heat exchanger surfaces.

According to another aspect of the invention the layer thickness of saidcoating on the shell and tube heat exchanger is 5-30 μm, preferably 2-20μm,

According to yet another aspect of the invention the coating comprisingsilicon oxide, SiO_(x), has an atomic ratio of O/Si ≧1.5-3, preferablyO/Si ≧2-2.5.

According to still another aspect of the invention the composition has acontent of carbon ≧20-60 atomic %, preferably ≧30-40 atomic %.

The shell and tube heat exchanger is advantageous in that fouling of thesurfaces is reduced significantly. By applying a coating compositioncomprising sol-gel material with organosilicon compounds to the shelland tube heat exchanger surfaces both the surface free energy androughness is lowered, leading to reduction of fouling and easy cleaningof shell and tube heat exchanger surfaces. Moreover, the sol-gel coatedshell and tube heat exchanger surfaces of the invention exhibit anexcellent wear resistance and have a flexibility that reduces the riskof cracks appearing in the coating. Furthermore, by the shell and tubeheat exchanger according to the invention it is possible to reduce theoverall dimensions of the heat exchanger while the heat transfercapacity of the shell and tube heat exchanger is maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the invention will appearfrom the following detailed description of different embodiments of theinvention with reference to the accompanying schematic drawings, inwhich

FIG. 1 is a schematic drawing of a shell and tube heat exchangeraccording to the invention,

FIG. 2 is a schematic cross section of a surface of a shell and tubeheat exchanger having an anti fouling coating according to theinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a side elevation of a shell and tube heat exchanger 1 arrangedin accordance with a preferred embodiment of the invention. The shelland tube heat exchanger 1 includes a shell 2 having an inlet end-cap 3attached to a first end 4 of the shell 2. An outlet end-cap 5 isattached to a second end 6 of the shell 2.

A cutaway portion of shell 2 reveals a tube bundle 7 housed within theshell 2. The tube bundle 7 includes a plurality of parallel-spaced tubes8 that traverse the interior of shell 2 from a first end to a second endof the tube bundle. A plurality of baffles 11 are arranged within theshell 2 and support the parallel-spaced tubes 8 of the tube bundle 7.

In operation, a first heat exchange fluid, such as a flue gas, or hotmedium carrying waste heat, or the like, is introduced to shell 2through an inlet. The first heat exchange fluid traverses the shell 2through a pathway created by the baffles and exits the shell 2 throughan outlet. A second heat exchange fluid, to be heated within heatexchanger 1 enters inlet end-cap 3 through an inlet. The second heatexchange fluid enters tube bundle 7 and is passed throughparallel-spaced tubes 8, while being heated by the first heat exchangefluid passing through the shell side of heat exchanger 1. The secondheat exchange fluid eventually passes from tube bundle 7 to outletend-cap 5 and exits heat exchanger 1 through an outlet tube.

The coating used according to the present invention may be referred toas a non-stick coating and makes it easy to clean the surfaces of afouled shell and tube heat exchanger. The coated surfaces according tothe present invention show a better heat transfer over time compared toconventional shell and tube heat exchanger surfaces since the latterones gets fouled much quicker and thus decrease the heat transferperformance to a larger extent. The coating of the surfaces also resultsin a much more even surface thus resulting in better flowcharacteristics. Also the pressure drop is reduced over time for a shelland tube heat exchanger according to the present invention in comparisonwith conventional shell and tube heat exchangers, since the buildup ofimpurities, microorganisms and other substances is not as pronounced.

The coated shell and tube heat exchanger according to the presentinvention may easily be cleaned just using high pressure washing withwater. With a surface according to the present invention there is noneed for extensive time consuming mechanical cleaning or cleaning usingstrong acids, bases or detergents, such as e.g. NaOH and HNO₃.

According to the present invention the surfaces of a shell and tube heatexchanger is coated with a composition comprising organosiliconcompounds using a sol-gel process. The organosilicon compounds arestarting materials used in the sol-gel process and are preferablysilicon alkoxy compounds. In the sol-gel process a sol is converted intoa gel to produce nano-materials. Through hydrolysis and condensationreactions a three-dimensional network of interlayered molecules isproduced in a liquid. Thermal processing stages serve to process the gelfurther into nano-materials or nanostructures resulting in a finalcoating. The coating comprising said nano-materials or nanostructuresmainly comprise silicon oxide, SiO_(x), having an atomic ratio ofO/Si >1, preferably an atomic ratio of O/Si ≧1.5-3, and most preferablyO/Si ≧2-2.5. A preferred silicon oxide is silica, SiO₂. The siliconoxideforms a three dimensional network having excellent adhesion to thesurfaces.

The coating of the present invention further has a content of carbonsuch as found in hydrocarbon chains. The hydrocarbons may or may nothave functional groups such as found in hydrocarbon chains or aromaticgroups, e g C═O, C—O, C—O—C, C—N, N—C—O, N—C═O, etc. Preferably thecarbon content is ≧10 atomic %, preferably ≧20-60 atomic %, and mostpreferably ≧30-40 atomic %. The hydrocarbons impart flexibility andresilience to the coating. The hydrocarbon chains are hydrophobic andoleophobic which results in the non-stick properties of the coating.

In FIG. 2 is shown a schematic drawing of a surface 9 for a shell andtube heat exchanger provided with a siliconoxide sol gel coating 10.Between the surface 9 itself and the siliconoxide layer is an interface11 between the coating siloxane and a metal oxide film of the surface 9.The coating bulk that follows said interface is the siloxane network 12with organic linker chains and voids that impart flexibility to thecoating. The outermost layer is a functional surface 13, i e ahydrophobic/oleophobic surface for fouling reduction.

By the combination of a durable and yet flexible coating, a surface fora shell and tube heat exchanger is achieved which has excellentnon-stick properties and also is wear and crack resistant. Theflexibility of the coating is especially important in order to avoidcracking of the coating when the surfaces move in relation to eachother.

In one embodiment of the present invention at least one sol comprisingorganosilicon compounds is applied to the surface to be coated. Thesurface may be wetted/coated with the sol in any suitable way. It ispreferable for the surface coating to be applied by spraying, dipping orflooding. At least a part of one side of the shell and tube heatexchanger surface is to be coated. Alternatively, all surfaces of atleast one side of a surface which during use in a shell and tube heatexchanger would be in contact with a fluid are coated. Also, at leastone side of a shell and tube heat exchanger surface may be entirelycoated. Alternatively, both sides of the tube may be coated. If bothsides are coated, they may be partly or fully coated, in anycombination. Naturally, more surfaces than the surfaces intended to bein contact with fluid may be coated. Preferably, all surfaces in contactwith a fluid giving rise to fouling are coated.

In another embodiment the method comprises a pretreatment of at leastthe surfaces on the heat exchanger tubes to be coated with at least onesol. This pretreatment is also preferably carried out by means ofdipping, flooding or spraying. The pretreatment is used to clean thesurfaces to be coated in order to obtain increased adhesion of thelatter coating to the heat exchanger tube. Examples of suchpretreatments are treatment with acetone and/or alkaline solutions, e.g.caustic solution.

In another embodiment the method comprises thermal processing stages,e.g. a drying operation may be carried out after a pretreatment and adrying and/or curing operation is often necessary after the actualcoating of the tube with said sol. The coating is preferably subjectedto heat using conventional heating apparatus, such as e.g. ovens.

The composition comprising SiOx is applied to a surface to be used in ashell and tube heat exchanger. The application of the composition isdone by means of sol-gel processing. The resulting film of saidcomposition on the surface is preferably between 1 and 30 μm thick. Thethickness of the coated film is important for the use in a shell andtube heat exchanger . A film thickness below 1 μm is considered beingnot enough wear resistant since the surfaces in a shell and tube heatexchanger in use are able to move slightly in relation to each other.This slight movement causes wear on the film and with time the coatingwill become worn down. Also the thickness of the film has an upper limitsince the application of substances on the heat transfer surfacesinfluences the heat transfer and thus the performance of the shell andtube heat exchanger. The upper limit for the applied film is preferably30 μm. Thus, the film thickness of the silicon oxide sol containingcomposition is 1-30 μm, preferably 1.5-25 μm, preferably 2-20 μm,preferably 2-15 μm, preferably 2-10 μm and preferably 3-10 μm.

The base material for the surfaces may be chosen from several metals andmetal alloys. Preferably, the base material is chosen from titanium,nickel, copper, any alloys of the before mentioned, stainless steeland/or carbon steel. However, titanium, any alloys of the beforementioned or stainless steel is preferred.

From the description above follows that, although various embodiments ofthe invention have been described and shown, the invention is notrestricted thereto, but may also be embodied in other ways within thescope of the subject-matter defined in the following claims.

EXAMPLES

In the search for prolonged operational time of off-shore equipment,tests were conducted on low surface energy glass ceramic coatings.

Two low surface energy glass ceramic coatings Coat 1 and Coat 2 weretested and the results are presented below. Coat 1 is a silan terminatedpolymer in butyl acetate and Coat 2 is a polysiloxan-urethan resin insolvent naphtha/butylacetate.

Phase A

The analysis documents the properties of coatings concerning substratewetting and adhesion, contact angle, coating thickness and stabilitytowards 1.2% HNO₃ in H₂O, 1% NaOH in H₂O and crude oil. The results aresummarized below in Table 1.

TABLE 1 Coat 1 Coat 2 Substrate Excellent Excellent wetting SubstrateAl: 0/0 Al: 0/0 adhesion Stainless steel: 0/0 Stainless steel: 0/0 Ti:0/0 (see below) Ti: 0/0 (see below) Contact angle H2O: 102-103° H2O:102-103° measurements Coating 4-10 μm 2-4 μm thickness Stability 1.2%HNO3 in H2O: 1½ h at 75° C. 1.2% HNO3 in H2O: 1½ h at 75° C. 1% NaOH inH2O: 3 h at 85° C. 1% NaOH in H2O: 2 h at 85° C. Crude oil: 6 months atRT Crude oil: 6 months at RT

Both coatings showed excellent wetting when spray coated onto eitherstainless steel or titanium substrates.

Adhesion was determined by cross-cut/tape test according to DIN EN ISO2409. Rating is from 0 (excellent) to 5 (terrible). 0 or 1 is acceptablewhile 2 to 5 is not. First digit indicates rating after cross cut (1 mmgrid) and the second digit gives rating after tape has been applied andtaken off again.

To obtain the best adhesion for Coat 1 and Coat 2 the substratesrequired pre-treatment.

To obtain the best adhesion of Coat 1 on stainless steel the substratemust be pre-treated. The substrate is submerged in an alkaline cleaningdetergent for 30 minutes. Afterwards the substrate is washed with waterand demineralized water and dried before Coat 1 is applied within halfan hour to achieve the optimal adhesion. Tests have shown the adhesionis reduced if cleaning of the substrate is only carried out withacetone. Pre-treatment is also necessary for stainless steel substratescoated with Coat 2. This coating displayed unaffected adhesion whetheran alkaline detergent or acetone was used as pre-treatment. If thepre-treatment step is neglected or not made correctly it will affectcoating adhesion.

Both coatings showed good stability under acidic condition. The coatingswere stable for 1½ hour at 75° C. and more than 24 hours at roomtemperature.

Under alkaline conditions Coat 1 showed a better result than Coat 2.Coat 1 could withstand the alkaline conditions for 3 hours at 85° C. andCoat 2 for 2 hours at 85° C. Both coatings showed no decomposition orreduction in oleophobic properties after being submerged for 6 months incrude oil at room temperature.

Phase B

Coating of Shell and Tube Heat Exchanger Surfaces

Coat 1 and Coat 2 were applied to a tube bundle. All tubes underwentpre-treatment which consisted of:

1. Submerging in liquid nitrogen (−196° C.)

2. Treatment with acidic and alkaline solutions to remove fouling

3. High pressure washing of the tubes with water

4. Assembly of the tube bundle for pressure testing

5. Disassembly of the tube bundle. Tubes left to dry before application

This pre-treatment was completed the day before Coat 1 and Coat 2 wereapplied to the tubes. Consequently, this procedure did not follow therecommended approach as outlined in Phase A. As the tubes have been leftto dry at ambient temperature, some tubes were still wet. 15 tubes weretreated with Coat 1 and the remaining 15 tubes with Coat 2 by spraycoating. The heat exchanger tubes were coated on both sides. The finalfilm thickness was measured to be 2-4 μm and the coating was applied onboth sides of the tubes. Curing/drying was performed at elevatedtemperatures of 200° C. or 160° C. respectively for 1½ hour in anon-site oven. Upon completion the coated heat exchangers were weighedand coating thickness was measured. It was observed that some tubes hadsome coating imperfections and small defects.

The heat exchanger tubes were then assembled with the remaininguntreated tubes. The coated tubes were placed respectively in the front,middle and end of the assembled. The evaluation of the coated tubes wasperformed after more than seven months of operation.

Phase C

Determination of Content in Coating by XPS Analysis

Three different silicon oxide-coated Ti substrates were analyzed beforeand after use by means of XPS (X-ray Photoelectron Spectroscopy), alsoknown as ESCA (Electron Spectroscopy for Chemical Analysis). The XPSmethod provides quantitative chemical information—the chemicalcomposition expressed in atomic %—for the outermost 2-10 nm of surfaces.

The measuring principle is that a sample, placed in high vacuum, isirradiated with well defined x-ray energy resulting in the emission ofphotoelectrons. Only those from the outermost surface layers reach thedetector. By analyzing the kinetic energy of these photoelectrons, theirbinding energy can be calculated, thus giving their origin in relationto the element and the electron shell.

XPS provides quantitative data on both the elemental composition anddifferent chemical states of an element (different functional groups,chemical bonding, oxidation state, etc). All elements except hydrogenand helium are detected and the surface chemical composition obtained isexpressed in atomic %.

XPS spectra were recorded using a Kratos AXIS Ultra^(DLD) x-rayphotoelectron spectrometer. The samples were analyzed using amonochromatic Al x-ray source. The analysis area was below 1 mm².

In the analysis wide spectra were run to detect elements present in thesurface layer. The relative surface compositions were obtained fromquantification of detail spectra run for each element.

The following three samples were XPS analyzed:

1. Siliconoxide (new) on Ti-plate—coating on both sides.

2. Siliconoxide (used) on Ti-plate—coating on one side

3. Siliconoxide on DIN 1.4401 stainless steel plate, coating on bothsides.

The analysis was performed in one position per sample, except for sample1, where two positions were analyzed. The results are summarized inTable 2 showing the relative surface composition in atomic % and atomicratio O/Si.

TABLE 2 Sample O/Si C O Si N 1 new (pt 1) 2.25 61.1 23.5 10.5 4.2 2 new(pt 2) 2.30 61.0 23.9 10.4 4.1 2 used 2.29 68.0 19.5 8.6 3.1 3 1.46 41.934.3 23.4  (0.2)* *weak peak in detail spectra, signal close to noiselevel

As seen in Table 2 mainly C, O and Si were detected on the outermostsurfaces, i e 41.9-68.0 atomic % C, 19.5-34.3 atomic % 0 and 8.6-23.4atomic % Si.

Note that in the atomic ratios O/Si, the total amount of oxygen is used.This means that also oxygen in functional groups with carbon isincluded. Otherwise for silica, from theory is expected a ratio O/Si of2.0 for the bulk pure silica SiO₂.

Inspection of Tubes after Operation

The term fouling is used to describe the deposits formed on the tubesduring operation. The fouling are residues and deposits formed by thecrude oil and consists of a waxy, organic part and a mineral/inorganicpart.

The visual inspection revealed that the tubes with the coatingdesignated Coat 1 was covered with the least amount of fouling on thecrude oil facing tube side. Also, the other coating system designatedCoat 2 had a reduced amount of fouling on the crude oil facing tube sidecompared to the bare titanium surface but to a lesser extent then Coat 1

By subtracting the average weight of a clean tube from the weightrecorded for the individual fouled tubes the average amount of foulingper surface type was calculated (table 3). Note, the weight of thecoating was not compensated for and so the real fouling reduction isslightly higher. If the coating is estimated to be pure SiO₂ (density2.6 g/cm³) then the amount of coating per tube is about 20 g.

TABLE 3 Average Fouling Surface fouling* (g) STDEV reduction (%)Titanium 585 125 — Coat 1 203 48 65 Coat 2 427 144 27

For both coating systems the fouling of the tubes were more easilyremoved compared to the fouling adhering to the bare titanium surface,see Table 4. The difference in cleaning requirements was tested bymanually wiping of the tubes with a tissue and by high pressure watercleaning. Just wiping the tubes with a tissue showed that the foulingwas very easily removed from the coated tubes, contrary to the uncoatedtubes. By using water jet all fouling except for one or two smallpatches could be removed from the Coat 1 coated surface. On the Coat 2coated surface some more fouling was present after water jet cleaning.This fouling had the appearance of slightly burnt oil.

Some loss of coating was observed in the contact points but overall thecoated surface that had been in contact with the crude oil was in a goodcondition.

On the sea water facing side both coatings had deteriorated and could bepeeled of quite easily.

TABLE 4 Coat 1 Coat 2 Non-coated View very little fouling reducedfouling fouling significant compared and widespread Wipe very easy tovery easy to fouling was not with remove fouling remove fouling removedtissue High the tubes most of the fouling even after attempts pressureappeared as new was removed of manual removal water of fouling, still awashing considerable layer remains

The coating tolerance to immersion in liquid nitrogen for gasket removalwas tested. One Coat 1 and one Coat 2 tube were treated in liquidnitrogen, at −196° C., to remove the rubber gaskets. The coatings didnot appear do suffer from the extreme temperature changes. Subsequentlythe tubes were washed by high pressure water, which removed almost allfouling. No coating delimitation or failure was observed for eithercoating system.

1. A shell and tube heat exchanger comprising: a shell having an inletend-cap attached to a first end of the shell, wherein an outlet end-capis attached to a second end of the shell and a tube bundle being housedwithin the shell, said tube bundle including a plurality ofparallel-spaced tubes that traverse the interior of shell from a firstend to a second end of the tube bundle, and wherein a plurality ofbaffles are arranged within the shell supporting the parallel-spacedtubes of the tube bundle, wherein said shell and tube heat exchanger isprovided with a coating comprising: silicon oxide, SiO_(x), having anatomic ratio of O/Si>1, a content of carbon ≧10 atomic % and a coatinglayer thickness of about 1-30 μm, which coating was prepared by sol-gelprocessing and applied to at least a part of the shell and tube heatexchanger surfaces.
 2. A shell and tube heat exchanger according to clam1, wherein the layer thickness of said coating on the surfaces is 1.5-25μm, preferably 2-20 μm, more preferably 2-15 μm, even more preferably2-10 μm, and most preferably 3-10 μm.
 3. A shell and tube heat exchangeraccording to claim 1, wherein the coating comprising: silicon oxide,SiO_(x), has an atomic ratio of O/Si ≧1.5-3, preferably O/Si ≧2-2.5. 4.A shell and tube heat exchanger according to claim 1, wherein thecomposition has a content of carbon ≧20-60 atomic %, preferably 30-40atomic %.